The invention relates to a raisable and lowerable elevating tower which extends downwards over the bridge edge and to which is fitted a work platform projecting beneath the bridge, and having a chassis located on the bridge and exhibiting a bearing element which is vertical in the working position and is connected to the elevating tower by means of arms protruding over the bridge edge.
A device of this kind is known, for example, from European Disclosure Specification 156 304. As a vertical bearing element, there is here provided a so-called guide tower of considerable height. On its side pointing, in the operating position, towards the bridge edge, it has guide rails, in which a slide is guided which can be moved up and down by means of a lifting cylinder. To the slide are fastened jibs projecting perpendicularly to the guide tower, i.e. horizontally, which are firmly connected to the elevating tower. The rigid structural unit comprising elevating tower, jibs and slide is thus guided on the guide tower. The vertical position of the jibs determines the vertical position of the work platform.
Often it is necessary to cross over high obstacles disposed on the bridge edge, e.g. protective fencing, soundproofing walls or lighting masts, with the jibs. Although this can be done by using a sufficiently high guide tower, the elevating tower can then no longer be moved in a vertical direction in order to bring the work platform to the correct distance from the underside of the bridge support.
A further problem presents itself in connection with so-called truss bridges, the girders of which located at both sides of the carriageway comprise mutually alternating vertical columns and oblique ties, which are connected to one another at the top by horizontal trusses. Here, one is forced to reach with the jibs through the windows formed from column, tie and truss, to be precise, over or under the oblique tie. Depending upon the pattern of the latticework, this necessitates, at various locations along the bridge, different heights for the work platform, which is unacceptable.